I used to buy Hills T/D dental dry food and give it as treats as well. The kibbles are really big (dog food sized almost) and they do a great job of knocking tarter off teeth because it takes cats a few good chews to crunch it up. I stopped giving it though because a few of my cats have other food related issues and I have to buy special foods for their special needs.

Medi-Cal has come out with their own feline dental dry food too and it comes in a light pink bag and also has nice big kibbles.